Direct Stepwise Oxidation of Methane to Methanol over CuâSiO2

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Direct Stepwise Oxidation of Methane to Methanol over Cu-SiO2 Selmi Erim Bozbag, Petr Šot, Maarten Nachtegaal, Marco Ranocchiari, Jeroen Anton van Bokhoven, and Carl Mesters ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01021 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Direct Stepwise Oxidation of Methane to Methanol over Cu-SiO2 Selmi E. Bozbag1,2,†, Petr Sot1,2, Maarten Nachtegaal1, Marco Ranocchiari1, Jeroen A. van Bokhoven1,2,*, Carl Mesters3,* 1

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Paul Scherrer Institute, Villigen, CH-5232 Switzerland.

ETH Zurich, Institute for Chemical and Bioengineering, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland. 3

Shell Technology Center Houston, 3333 Highway 6 South, Houston, TX 77083, USA

KEYWORDS: Methane to methanol, silica, copper, activation temperature, XRD, XAS.

ABSTRACT: Cu supported on SiO2 can be used to directly convert methane to methanol in a step-wise process with no intrinsic need for a zeolite support. Effects of parameters such as the O2 activation temperature, activation time, CH4 reaction temperature, CH4 partial pressure (pCH4) and Cu wt. % on methanol yield were investigated. Increasing the O2 activation temperature in the 200-800 °C range significantly improved the methanol yield and when carried out at 800 °C, a methanol yield of 11.5 µmol/gcatalyst was obtained after reaction with methane at 200 °C for the sample with 2 wt. % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol with decreased Cu wt. % from 30 to 1, respectively. The increase in the O2 activation time also

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strongly influenced the yield which corresponded to the increase in yield by a factor of >2 between 1h and 8h. Increasing pCH4 from 0.05 to 1 atm resulted in a 5-fold increase in yield after activation at 450 °C however it resulted in at least 20% lower yields after activation at 800 °C showing that active sites of different nature were formed at different activation temperatures. The increase in yield with ramped O2 activation temperature correlated with the dehydration of the samples as evidenced by X-ray absorption near edge spectroscopy (XANES) and via mass spectroscopy (MS) traces of H2O during the O2 activation step.

1. Introduction Direct conversion of methane into methanol using molecular oxygen remains one of the greatest challenges in catalysis. Although this reaction is thermodynamically possible, it is difficult to omit the kinetically controlled over-oxidation of methane since the desired product methanol is more reactive. This problem can be addressed by forming a stable complex of the reaction intermediate thereby preventing the total oxidation of methane to CO2 and then converting the intermediate to methanol in a step-wise process1-3. Over the last decade a number of reports showed the potential of Cu loaded zeolites such as ZSM-5 4, mordenite 5, small-pore zeolites 6-8 and others 9 for the conversion of methane to methanol. The stepwise (sub)stoichiometric process consists of the activation of O2 on active copper-oxo sites 10-14 followed by the reaction of methane in which the oxidized copper sites react with methane to form a chemisorbed stable intermediate which allows the production of methanol via the subsequent hydration step either at process conditions 5, 15 or via room temperature solvent extraction 16. Processes using oxidants other than O2 17-21 and a catalytic continuous process were also recently reported 22. At ambient pressure, the formation of the


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active copper-oxo species requires the dehydration of the sample in a high temperature treatment typically >400 °C23-24. At high temperature, the chemisorbed methane intermediate decomposes and therefore the subsequent reaction step is usually carried out at lower temperatures (400 °C, mono(µ-oxo)dicopper in Cu-ZSM5 43 and Cu-SSZ13&39 7, trans-µ-1,2-peroxo dicopper(II) in Cu-SSZ-13&39, tricopper in Cu-HMOR 25 and tetraneutral clusters in Cu-MOR 40 were proposed as active sites. Papas et al. showed that the


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hydrated Z[CuIIOH] monocopper sites were the precursors to the active sites in Cu-SSZ-13 and the activity correlated with the concentration of reducible Cu species 8. Although absent in biological systems, the mono copper core is a thermodynamically favored species in zeolites 23. Alayon et al. proposed an additional water-stable active Cu species for Cu-MOR formed upon O2 activation at 450 °C followed by hydration at 200 °C 44. When the step-wise process was carried out isothermally at 200 °C and the subsequent CH4 reaction step was carried out at high pressure, higher methanol yields were obtained and nanoparticles were proposed as the active sites based on ultraviolet-visible spectroscopy (UV-Vis) and transmission electron microscopy (TEM) data45-46. The complicated formation mechanisms of active copper-oxo sites, which can result in a variety of active or inactive Cu species on zeolites, prompted us to investigate supports other than zeolites. For the partial oxidation of methane, Cu-SBA15 catalyst showed appreciable formaldehyde selectivity at low CH4 conversions at 625 °C 47. Among other supports, amorphous SiO2 could be an attractive alternative to synthetic zeolites. Cu-SiO2 using a modified SiO2 with an Al2O3 content of 0.6 wt% (Si/Al = 141) did not show appreciable activity for the conversion of methane to methanol 4 most probably due to the absence of active sites which did not form on SiO2 at O2 activation temperatures below 500 °C. Active Cu-oxo sites might be formed on the silica surface at different conditions i.e. at higher O2 activation temperatures. In this study, we show that the direct step-wise conversion of methane to methanol is possible on Cu-SiO2 subsequent to the O2 activation at higher temperatures (> 500 °C) without the necessity of a zeolite support. The effects of activation temperature, activation duration, CH4 partial pressure during reaction and Cu wt. % on the methanol yield were investigated. Cu-SiO2 catalysts were characterized using N2 adsorption, X-ray diffraction (XRD), temperature programmed desorption-mass spectroscopy (TPD-MS), UV-Vis , in-situ X-ray absorption


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spectroscopy (XAS) and Scanning transmission electron microscopy (STEM) during or after various stages of the catalytic process. 2. Experimental 2.1.Catalyst Preparation and Activity Cu-SiO2 with varying Cu loadings (1,2,5,15,30 wt. %) were prepared via pore-volume impregnation of a silica support (PQ Silica PD44011) with an ammonium copper carbonate stock solution. The ammonium copper carbonate solution was prepared by adding 250 g of (NH4)2CO3 to 690 g of an ammonia solution (25%). After stirring for 30 min at 30 °C, a clear solution was obtained. Subsequently, 300 g of copper hydroxy carbonate was added, in portions of 60 g within 5 min to the vigorously stirred ammonia carbonate solution. The solution was then diluted to the desired metal content and impregnated to the silica support followed by drying at 150 °C and calcination at 400 °C. The drying and calcination steps were carried out in air. The activity tests were carried out in a continuous-flow fixed-bed reactor. The catalyst powder (0.7 g) was placed in the middle of a quartz tube reactor which was fitted into a tubular furnace. Gas flows were regulated using calibrated mass flow controllers (Bronkhorst). Figure 1 shows the two different reaction protocols employed. All gas flows were kept at 30 mL/min (1 atm, 25 °C). The first reaction protocol is hereafter referred to as R1: A stream of O2 was introduced into the reactor and the reactor was heated to the desired activation temperature with a ramp of 1 °C/min and remained at this temperature for a desired period. This was followed by cooling and removing the excess O2 with a He stream for 30 min. A CH4 stream (5% in He) was then introduced for 30 min. Finally, the system was cooled down to room temperature under He flow for methanol quantification by extraction (described below). The second reaction protocol


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(hereafter referred to as R2) was similar to that of the first one except methanol was extracted using steam at 200 °C just after the CH4 reaction step (Step G2 in Figure 1). Steam was supplied using a controlled evaporator mixer (CEM W202A, Bronkhorst) with a rate of 10 g/h where He (30 ml/min) was used as carrier gas. The evaporator temperature was set to 120 °C. All the lines between the evaporator and reactor were heated to 120 °C to avoid condensation. A condenser, cooled down to 15 °C, was connected at the outlet of the reactor where the extract was condensed. Liquid samples containing a mixture of water and methanol were taken from the bottom of the condenser at desired intervals. Once the extraction was completed (as analyzed by gas chromatography (GC), Agilent 6890), a second cycle was initiated with heating from 200 °C in O2 to the desired activation temperature. Reactor downstream concentrations were monitored using a quadrupole mass spectrometer (Pfeiffer, SEM voltage 1050 V, CH-TRON detector). 2.2. Methanol quantification In the first reaction protocol the amount of produced methanol was quantified by extraction with liquid water (Step G1). After the reaction with CH4 and cooling, the catalyst was transferred into a vial along with a magnetic stirring bar and 2 mL MilliQ water was added. The vial was closed and the mixture was stirred for 2 h. The suspension was filtered through a glass-fiber filter. Afterwards, 10 µl acetonitrile (internal standard, 10% V/V solution in water) was added and the mixture was analyzed on an Agilent 6890 GC equipped with a Restek Rtx®-5 column (30 m, ID = 0.25 mm, film thickness 0.25 µm) and a flame ionization detector. The oven temperature was held at 38 oC for 20 min and subsequently raised to 200 °C at 50 °C/min and held there for 7 min. Solvent extraction was repeated at least twice until no more methanol was obtained. The experimental error in methanol yield was ± 1 µmol/gcatalyst. In the second reaction


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protocol, after the liquid samples were taken from the outlet of the condenser (Step G2), acetonitrile was added as internal standard to the sample and the mixture was analyzed by GC.

2.3.Characterization of the catalysts Textural properties of the samples were determined by N2 adsorption using a Micromeritics Tristar II 3020 instrument. Approximately 100 mg of each material was transferred into the measurement tube, degassed at 50 mTorr at 150 °C overnight and analyzed. The total pore volume (Vtotal) was obtained using the uptake amount at P/P0 = 0.986 and micropore volume (Vmicro) was obtained using the t-plot method. The Brunauer, Emmett and Teller (BET) surface areas of the samples were also measured. XRD measurements were carried out using a Cu Kα source in a Bruker D8 Advance diffractometer. UV-Vis measurements were carried out in a measurement cell which was placed inside a glove bag filled with Ar. After treating the samples at various reactive steps described in Fig. 1, Ar was led to flow through the catalyst bed. Then, the quartz tubular reactor containing the catalyst bed was quickly removed from the test rig and transferred into the glove bag. Catalyst was then transferred into PMMA cuvettes and spectra were measured. The UV probe (reflection/backscattering, QR400-7-SR-125F, OD=3.18mm), connected by an optical fiber to the spectrometer, was positioned at the center of the PMMA cuvette which was fitted on a holder to obtain a good signal to noise ratio. The measurement cell was placed in a dark box to minimize stray light. UV-Vis spectra were recorded by a Maya 2000 Pro (Ocean Optics, Spectra Suite 2000) spectrometer at a scan rate of 200 ms per spectrum from 35000 cm-1 to 10000 cm-1. A thousand scans were averaged to produce a single spectrum. A spectrum of the bare SiO2 was used as reference. The morphologies of the catalysts were


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characterized by STEM on the aberration-corrected HD-2700CS (Hitachi; cold-field emitter), operated at an acceleration potential of 200 kV. A probe corrector (CEOS) is incorporated in the microscope column between the condenser lens and the probe-forming objective lens providing excellent high-resolution capability (beam diameter ca. 0.1 nm in the selected ultra-high resolution mode). Images (1024 x 1024 pixels) were recorded with a high-angle annular dark field (HAADF) detector with frame times of ca 15 s. These imaging conditions give rise to atomic number (Z) contrast, a highly sensitive method to detect even atoms of strongly scattering elements (high Z) on light supports. TEM was performed on a F30 microscope (FEI; field emission gun), operated at an acceleration potential of 300 kV. XAS spectra around the Cu Kedge were collected at the SuperXAS beamline, Swiss Light Source (SLS), of the Paul Scherrer Institute in Villigen, Switzerland. The SLS ring is operated in top up mode at 400 mA and 2.4 GeV. The polychromatic beam from the 2.9 Tesla superbend source was collimated with a Sicoated collimating mirror at 2.5 mrad. This also served to remove higher harmonics. The X-ray beam was subsequently monochromatized by a channel-cut monochromator using a Si(111) crystal pair. Focusing of the beam to a spot size of 500 x 100 (H x V) micrometer2 was achieved using a Rh-coated toroidal mirror. Spectra were measured in transmission mode using 15 cm long ion chambers filled with 20% He/ 80% N2 mixture resulting in about 8% absorption in both. A Cu foil mounted in front of a third ionization chamber filled with 100% N2 at 1.5 bar was measured simultaneously and used for absolute energy calibration. Around 15 mg of Cu-SiO2 was fixed between two plugs of quartz wools and placed in a 3 mm diameter, thin-walled quartz capillary reactor. This was affixed to an air blower set-up for controlled heating. The temperature inside the capillary reactor was pre-calibrated against the readout temperature of the blowing hot air with a thermocouple inserted in the reactor until the thermocouple's tip touched the middle of


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the catalyst bed. Subsequently, the thermocouple was removed during the experimental measurements and the air blower was set against the calibrated settings.

Figure 1. Schematic diagram of the thermal and chemical treatments on Cu-Silica samples during one catalytic cycle. A–D: activation; E: He purge; F: CH4 reaction; G: hydration and methanol production. (TO2 and TCH4 represent the O2 activation and CH4 reaction temperatures, respectively). 3. Results and Discussion 3.1.Catalytic Activity The samples screened in this study were 1, 2, 5, 15 and 30 wt. % Cu containing Cu-SiO2 which are hereafter referred to as Cu-SiO2-1, Cu-SiO2-2, Cu-SiO2-5, Cu-SiO2-15 and Cu-SiO2-30, respectively. All of the methanol yields reported were obtained using reaction protocol R1 unless otherwise mentioned. The formation of active copper-oxo species in zeolites is significantly influenced by the O2 activation temperature. Therefore, the effect of the activation temperature on the methanol yield was studied (Fig. 2). For both Cu-SiO2-2 and Cu-SiO2-5 very low methanol yields (0.7 µmol/gcatalyst, measured via G1) were obtained up to an activation


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temperature of 450 °C followed by exposure to CH4 (5% CH4 in He) at 200 °C. Methanol yield increased significantly as the O2 activation temperature increased up to 800 °C reaching 11.5 and 7.9 µmol/gcatalyst for Cu-SiO2-2 and Cu-SiO2-5, respectively. The yield obtained from the CuSiO2-2 is even higher than that reported for many Cu-zeolite systems4, 9. The reusability of the Cu-SiO2-2 in a subsequent catalytic cycle was also tested using reaction protocol R2 and resulted in a very similar methanol yield (Fig. 2). When a reaction cycle using Cu-SiO2-5 with O2 activation at 800 oC is followed by a reaction cycle with O2 activation at 450 oC, the methanol yield drops from 7.9 µmol/gcatalyst in the 1st cycle to 1.9 µmol/gcatalyst in the 2nd cycle showing the importance of high temperature activation. Data in Fig. 2 also shows the effect of methane partial pressure on the methanol yield. The increase of methane partial pressure from 0.05 to 1 atm resulted in an increase of the methanol yield from 0.73 and 0.5 to 4.8 µmol/gcatalyst for both catalysts after O2 activation at 450 °C for the Cu-SiO2-2 and Cu-SiO2-5, respectively. For the CuSiO2-2, the increase of partial pressure increased the yield after O2 activation at 650 °C but decreased the yield after O2 activation at 800 °C.


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Figure 2. Effect of O2 activation temperature (for 8 hours) on methanol yield (CH4 reaction was carried out at 200 °C and the pCH4 denotes the partial pressure of CH4 during Step F of the catalytic cycle).

Figure 3. Effect of (a) activation time and (b) CH4 reaction temperature on the methanol yield. Activation was carried out in O2 at 800 °C. Fig. 3a shows the effect of the activation time on the methanol yield when the activation was carried out at 800 °C. Data clearly indicate an increase in methanol yield with activation time up to 8 h where the yield increased from 3.5 to 9.5 µmol/gcatalyst for Cu-SiO2-5. The yield did not


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change considerably after an activation time of 12h. Cu-SiO2-1 and Cu-SiO2-2 showed a similar trend. Thus, with increased O2 exposure time, the number of reactive sites increased. The temperature of the CH4 reaction step has also an effect on the yield (Fig. 3b). The yield increased from 0.2 to 11.5 µmol/gcatalyst as the CH4 reaction temperature was raised from 50 to 200 °C for Cu-SiO2-2. As the CH4 reaction temperature was further increased to 300 °C, the yield dropped to 5.6 µmol/gcatalyst due to over-oxidation and the decomposition of the reacted methane intermediate. Similar results were also obtained for Cu-chabazite where CH4 reaction temperatures higher than 200 oC resulted in decreased methanol yield and decreased selectivity 8. Previous reports on Cu-zeolites showed that the Cu loading is an important parameter that affects the methanol yield. Cu loading in zeolites affect the formation of active sites i.e. at lower loadings (typically Cu/Al

Direct Stepwise Oxidation of Methane to Methanol over CuâSiO2

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